Preplaced aggregate concrete differs from conventional concrete in that 

 it contains a higher percentage of coarse aggregate in the finished product. 

 Because of point-to-point contact of the coarse aggregate, as placed, 

 drying shrinkage is about one-half the magnitude of that which normally 

 occurs in conventional concrete {U.S. Army Engineer Waterways Experiment 

 Station (WES), 1954; Shideler and Litvin, 1964}. 



The higher percentage of coarse aggregate in the concrete has an in- 

 fluence on the modulus of elasticity which is slightly higher than that of 

 conventional concrete. The other physical properties also appear to be more 

 affected by the properties of the coarse aggregate than occurs with conven- 

 tional concrete. In summary, the physical properties of preplaced aggregate 

 concrete are similar to those of conventional concrete except that overall 

 drying shrinkage of the former is considerably less. Accordingly, with a 

 properly proportioned and tested grout mix and with good construction 

 practices, allowable working stresses used for conventional concrete struc- 

 tural design may be used (U.S. Army Engineer, WES, 1954). 



The economics of its use are a function of site conditions and job 

 requirements. Structural forms for the concrete are usually more expensive 

 than that required for conventionally placed concrete because greater care 

 is needed to prevent grout leaks and placements usually require additional 

 lateral support. However, in underwater construction, higher placing rates 

 have been achieved by this method than by conventional placing methods. 



a. Types of Grouts . Slurries of Portland cement and water, with or 

 without sand, have long been used in the construction industry for filling 

 of rock fissures. Unless sufficient pressure is applied to squeeze out 

 excess water, settlement of solids may result in incomplete filling of 

 voids. Clean sand-cement or soil-cement slurries may be used for low- 

 pressure backfill grouting of rubble or rockfill where strength is not an 

 important consideration. 



As concrete technology has changed, basic grouts composed of Portland 

 cement, sand and water have been modified to more effectively produce 

 structural preplaced aggregate concrete. Such grouts may be modified 

 chemically by the inclusion of admixtures such as pozzolans, fluidifiers, 

 expansion agents, air-entraining agents, and coloring additives; or the 

 grout may be modified mechanically by use of specially designed high-speed 

 mixers . 



b. Grout and Aggregate Materials . 



(1) Cement . Grout can be made with any one of the types of cement 

 that complies with ASTM, Standard C150, Corps of Engineers specification 

 CRD-C 201, which would be suitable for use in conventional concrete and 

 produce the required conditions for preplaced aggregate concrete. The type 

 of cement should be selected in accordance with controlling factors, job 

 conditions, and service exposures which would influence the same selection 

 for conventional concrete. 



(2) Coarse Aggregate . Coarse aggregate must be clean, free of 

 surface dust and fines, sound, durable, and should conform to ASTM Standard 

 C33, Corps of Engineers specification CRD-C133, for aggregate acceptance, 



184 



